Back in the ToAU merit days I mentioned it back then. I just remembered it now.
You gain keyitems when you reach high number in chains.
At the time of ToAU these keyitems would boost exp by 1-3%% depending on how many you collected.
Since Abyssea is made exp kinda unfair I support that these exp boosting keyitems should be added such as,
Edit: 50th exp chain +1% exp keyitem
1Conquest exp boost+3%
2Conquest exp boost+3%
3Conquest exp boost+3%
4Conquest exp boost+3%
1wotg exp boost+2%
2wotg exp boost+2%
3wotg exp boost+3%
4wotg exp boost+3%
1toau+3%
2toau+2%
3toau+3%
4toau+2%
These keyitems would be obtainble when you reached the... 100th,200th,300th,400th, exp chain. Also every 100th chain should be rewarded with 10000exp,20000exp,30000exp,40000exp and roughly earn boost to npc points such as conquest,imperial etc.
Since Abyssea still wins even with these adjustments added there should be adjustments to the exp cap or progression with light growth or simplely add a mim enter cap at lv70 to add balance to the game.
Exp chain achivments is still a good idea but might not be good for boosting exp idea. Instead it may be better to earn more npc points per kill with every 100th chain you earn.